1. Field of the Invention
The present invention relates to a collimator to be used in a nuclear medical apparatus such as a SPECT (Single Photon Emission Computed Tomography) apparatus, and a method for manufacturing such a collimator.
2. Description of the Background Art
In a nuclear medical apparatus such as a SPECT apparatus, .gamma. rays emitted from radioactive materials deposited inside a body to be examined are detected, and an image of a distribution of the radioactive materials inside the body is obtained on a basis of the detected .gamma. ray signals, where the obtained image is utilized in the diagnosis of a cancer and a tumor. In such a nuclear medical apparatus, a collimator is attached on a detector device in order to selectively collect the .gamma. rays from the radioactive materials inside the body at the detector device. The .gamma. rays selectively collected at the detector device by using the collimator are then converted into light signals and then into electric signals by using a scintillator, and the obtained electric signals corresponding to the detected .gamma. rays are utilized as the image data in the image reconstruction process.
For such a collimator to be used in a nuclear medical apparatus, there are several types including a parallel hole collimator in which all the holes arranged in an array are parallel to each other, and a single focus (fan beam) collimator in which each hole in an array is provided with a prescribed inclination angle such that the collimator as a whole has a focal line in order to improve the sensitivity and the resolution of the collimator.
In the SPECT apparatus for the head portion diagnosis, three such collimators are used in an arrangement in which each collimator is located on each side of an equilateral triangle formed by detectors arranged around the head portion of a patient.
Among such a collimator to be used in the SPECT apparatus, the single focus collimator has an increasing demand in recent years because of its usefulness in the SPECT apparatus for brain, but has been rather difficult to manufacture with high precision conventionally, because each hole in the array must be manufactured to be oriented toward a single focal line. This single focus collimator has usually been manufactured by the following manufacturing method using pins.
Namely, in the conventional manufacturing method using pins, approximately thirty to fifty thousand pins each in a shape of a hole of a collimator to be manufactured are mounted between two templates with pre-manufactured pin positions in an array such that all the pins are oriented toward a predetermined single focal line, and then the lead is casted between the templates with the pins mounted, such that a desired single focus collimator body with all the holes arranged in an array oriented toward the predetermined single focal line can be obtained by pulling out all the pins after the lead casting.
However, such a conventional method of manufacturing a single focus collimator has been associated with the problem that the precision of the manufactured single focus collimator is often deteriorated by the bending of the very thin templates due to the weights of the pins, and by the inaccuracy of the pin orientation due to the looseness of the fitting of the pins at the pin positions on the templates. In particular, the precision of the single focus collimator manufactured by this conventional method using pins has been rather poor because of the poor manufacturing precision due to the above described reasons and of the deformation due to the heat produced in the lead casting process, such that the focal line is often not precisely focused. When such a single focus collimator having not precisely focused focal line is used in the SPECT apparatus, the image quality of the SPECT image obtained by the SPECT apparatus is deteriorated considerably.
The similar problem also existed for a conventional cone beam collimator in which the focal point is often not precisely focused.
In general, the collimator manufactured by using the metal casting process has a poor focus precision.